Copolymer of ethylene and of at least one alpha-olefin and method for obtaining same

ABSTRACT

Copolymer of ethylene and of at least one alpha-olefin obtained by means of a catalytic solid base on chromium oxide deposited on an inorganic support, the said copolymer having a melt flow index HLMI, measured in g/10 min, an amount of alpha-olefin Q, pressed in g of alpha-olefin per kg of copolymer, a standard density SD, measured in kg/m 3 , at least equal to (952.75+5.40×log HLMI−0.79×Q) and a relatively broad molecular mass distribution, characterized by a ratio of the dynamic viscosities μ 0 /μ 2  at least equal to (23.67−6.67×log HLMI).

The present invention relates to copolymers of ethylene and of at leastone alpha-olefin exhibiting an advantageous combination ofcharacteristics which renders them particularly suited to processing byextrusion and extrusion-blow moulding and more particularly to themanufacture of articles exhibiting good resistance to stress cracking.It also relates to a process for producing these copolymers.

It is known that the resistance to cracking of a polyethylene increasesby incorporating an alpha-olefin therein. However, the maximum amount ofalpha-olefin which can be incorporated in polyethylene is limitedbecause its incorporation results in a decrease in the standard densityof polyethylene and consequently a decline in the mechanical properties,such as the stiffness. Furthermore, when, in the presence of a chromiumcatalyst, a small amount of alpha-olefin is incorporated in polyethylenethe incorporation does not always occur homogeneously, in particularwith regard to chains of high molecular mass.

U.S. Pat. No. 5,236,998 provides a solution to this problem by preparingan ethylene copolymer comprising three polyethylene fractions, of whichonly the fraction of high molecular weight comprises the alpha-olefin,by means of a Ziegler-Natta catalyst in two reactors arranged inparallel, the first of which only comprises ethylene, at hightemperature, and the second of which comprises an ethylene/alpha-olefinmixture, at lower temperature, and then the polymerization is continuedin a third reactor, in which the polymers resulting from the tworeactors are combined. Such a process is difficult to carry outindustrially and produces a heterogeneous mixture of resins comprisingresidual chlorine originating from the catalyst.

Furthermore, it is known that copolymers comprising fewer catalyticresidues and more particularly comprising very little chlorine can beobtained by means of catalysts based on chromium oxide deposited on asupport (generally known as chromium catalysts). It is also known thatethylene polymers obtained with catalysts of this type have a bettermelt strength than polymers obtained by means of Ziegler-Nattacatalysts. However, the use of conventional chromium catalysts in aprocess for the copolymerization of ethylene and of at least onealpha-olefin in a single stage does not make it possible to incorporatethe alpha-olefin homogeneously in the copolymer and more particularly inthe chains of higher molecular weight. In addition, the copolymersproduced by means of conventional chromium catalysts do not have, for agiven melt flow index, both a high standard density and a highalpha-olefin content. Furthermore, the copolymers produced by means ofconventional chromium catalysts in single-stage polymerization processeshave a relatively narrow distribution of molecular masses and a ratio ofthe dynamic viscosities μ₀/μ₂ which are only adjustable within narrowlimits.

The present invention is targeted at overcoming the abovementioneddisadvantages by providing ethylene copolymers, obtained by means of acatalytic solid based on chromium oxide, which exhibit, for a given meltflow index, a better compromise between the standard density and thealpha-olefin content and which also exhibit a relatively broaddistribution of the molecular masses.

The invention consequently relates to a copolymer of ethylene and of atleast one alpha-olefin obtained by means of a catalytic solid based onchromium oxide, the said copolymer having a melt flow index HLMI,expressed in g/10 min, an amount of alpha-olefin Q, expressed in g ofalpha-olefin per kg of copolymer, a standard density SD, expressed inkg/m³, and dynamic viscosities μ₀ and μ₂ measured at 190° C.,respectively at rate gradients of 1 and 100 s⁻¹, corresponding to therelationships

 SD≧(952.75+5.40×log HLMI−0.79×Q),

and

μ₀/μ₂≧(23.67−6.67×log HLMI).

For the purposes of the present invention, the term “copolymers ofethylene and of at least one alpha-olefin” is understood to denotecopolymers comprising monomer units derived from ethylene and monomerunits derived from one or more alpha-olefins and comprising at least90%, in particular at least 95%, by weight of monomer units derived fromethylene. The copolymers according to the invention preferably compriseat least 97% by weight of monomer units derived from ethylene. Thecopolymers composed essentially of monomer units derived from ethyleneand of monomer units derived from one or more alpha-olefins areparticularly preferred.

The alpha-olefins are generally chosen from those comprising from 3 to12 carbon atoms, more particularly from those comprising from 3 to 8carbon atoms. Good results have been obtained with 1-butene and/or1-hexene. The copolymers of ethylene and of 1-hexene are veryparticularly preferred.

For the purposes of the present invention, the alpha-olefin content Q isexpressed in g of alpha-olefin per kg of copolymer. It is measured by¹³C NMR according to the method described in J. C. Randall, JMS-Rev.Macromol. Chem. Phys., C29(2&3), p. 201-317 (1989), that is to say thatthe content of units derived from the alpha-olefin is calculated fromthe measurements of the integrals of the lines characteristic of thealpha-olefin with respect to the integral of the line characteristic ofthe units derived from ethylene (30 ppm).

The alpha-olefin content in the copolymer according to the invention isgenerally at least 1 g per kg of polymer, in particular at least 4 g/kg,values of at least 6 g/kg being favourable. The alpha-olefin content isusually at most 100 g/kg, preferably at most 50 g/kg, of polymer. Analpha-olefin content which does not exceed 30 g/kg is particularlypreferred.

One of the essential characteristics of the copolymer according to theinvention is therefore that it exhibits, for a given melt flow indexHLMI and a given alpha-olefin content Q, a higher SD than known ethylenecopolymers. In the context of the present invention, the standarddensity SD is measured according to ASTM Standard D 792. The SD ismeasured on a sample prepared according to ASTM Standard D 1928,Procedure C. The SD of the copolymer according to the invention ispreferably at least equal to (952.75+5.40×log HLMI−0.70×Q). Copolymersexhibiting an SD at least equal to (952.75+5.40×log HLMI−0.63×Q) areparticularly preferred.

The copolymers according to the invention usually exhibit an SD ofgreater than 935 kg/m³. Copolymers which have given good results arethose in which the SD is at least equal to 940 kg/M³, more particularlythose in which the SD is at least equal to 945 kg/m³. The SD of thecopolymers according to the invention generally does not exceed 965kg/M³ and it preferably does not exceed 962 kg/m³. Copolymers in whichthe SD does not exceed 959 kg/M³ are particularly preferred.

The copolymers according to the invention generally exhibit a melt flowindex HLMI, measured at 190° C. under a load of 21.6 kg according toASTM Standard D 1238, Condition F (1986), which does not exceed 100 g/10min and generally does not exceed 50 g/10 min. The HLMI is generally atleast 0.1 g/10 min, in particular at least 0.5 g/10 min.

Another essential characteristic of the copolymers according to theinvention is their relatively broad distribution of molecular masses,characterized by a ratio μ₀/μ₂, between the dynamic alpha-olefin andprocess for its production viscosities μ₀ and μ₂ measured at 190° C.,respectively at rate gradients of 1 and 100 s⁻¹, of greater than orequal to (23.67−6.67×log HLMI). In the context of the present invention,the dynamic viscosity μ₂ is determined by extrusion of the polymer at190° C. through a die with a length of 15 mm and a diameter of 1 mm at aconstant rate corresponding to a rate gradient of 100 s⁻¹ and bymeasuring the force transmitted by the piston during its descent. Thedynamic viscosity μ₂ is then calculated by the relationship μ₂=233×Fp,in which Fp represents the mean force exerted by the piston during themeasurement period of 30 seconds, expressed in daN. The cylinder and thepiston of the rheometer used for this measurement correspond to thecriteria of that used for the measurement of the melt flow indexaccording to ASTM Standard D 1238 (1986). In the context of the presentinvention, the dynamic viscosity μ₀is determined by the extrapolation,to a rate gradient of 1 s⁻¹, of the dynamic viscosity measurementscarried out at rate gradients of 7 to 3000 s⁻¹ according to the methoddescribed above for μ₂

The copolymers according to the invention generally exhibit a ratioμ₀/μ₂ at least equal to 10, more particularly at least equal to 12. Theratio μ₀/μ₂ usually does not exceed 50.

Furthermore, the ethylene copolymers according to the invention have adie swell (DS) which is generally at least 1.3 and preferably at least1.45. Copolymers which exhibit a DS of at least 1.55 are particularlypreferred. In the context of the present invention, the die swell isdetermined by extrusion of the polymer at 190° C. and at a rate gradientof 100 s⁻¹ through a die with a length of 15 mm and with a diameter of 1mm at a constant rate and by measuring the displacement of the pistonnecessary to extrude a 70 mm rod. The die swell is then calculated bythe relationship DS=0.5707{square root over (e)}, in which e representsthe displacement of the piston, expressed in mm. The cylinder and thepiston of the rheometer used for this measurement correspond to thecriteria of that used for the measurement of the melt flow indexaccording to ASTM Standard D 1238 (1986).

The copolymers according to the invention are generally characterized byhomogeneous incorporation of the alpha-olefin in the chains of differentmolecular mass. The homogeneity of the incorporation of the alpha-olefincan be characterized by the fractionation of the copolymer and by thedetermination of the alpha-olefin content for each fraction of differentmolecular mass. The graph in FIG. 1 gives, on the ordinate, thealpha-olefin content for, on the abscissa, fractions of differentmolecular masses present in a copolymer according to the invention. Thecopolymers according to the invention generally comprise fractions withmolecular masses varying from 5000 to 500,000, each comprising analpha-olefin content, expressed in g/kg, of 0.75×Q to 1.25×Q, Q beingthe alpha-olefin content of the copolymer.

The copolymers according to the invention are usually characterized byhigh incorporation of the alpha-olefin in the chains of high molecularmass. The copolymers according to the invention generally comprisefractions with molecular masses varying from 100,000 to 500,000, eachcomprising an alpha-olefin content of greater than 0.8×Q, moreparticularly of greater than 0.9×Q.

The copolymers according to the present invention generally compriseless than 10 ppm of chlorine. They generally comprise less than 5 ppm ofchlorine.

The present invention also relates to a process which is particularlywell suited to the preparation of the copolymers of ethylene and of atleast one alpha-olefin described above. This process consists inpolymerizing, in a single stage, ethylene and optionally one or morealpha-olefins as defined above in the presence of a catalytic solidbased on chromium oxide deposited on a homogeneous and amorphoussupport, comprising at least two constituents chosen from silica (X),alumina (Y) and aluminium phosphate (Z), and of a cocatalyst chosen fromorganoboron compounds.

The catalytic solid based on chromium oxide deposited on a homogeneousand amorphous support, comprising at least two constituents chosen fromsilica (X), alumina (Y) and aluminium phosphate (Z), is known and hasbeen disclosed in Patent Applications WO 94/26790, WO 94/26791 andEP-A-0,712,868, the contents of which are incorporated by reference. Itis generally obtained by impregnation or by mixing of the support with achromium compound, followed by activation at a temperature of 400 to1000° C. in an oxidizing atmosphere, so as to convert at least a portionof the chromium to hexavalent chromium.

The amount of chromium present in the catalytic solid is generally from0.05 to 10% by weight, preferably from 0.1 to 5% by weight, moreparticularly from 0.25 to 2% by weight, with respect to the total weightof the catalytic solid.

The support advantageously exhibits a specific surface (SS), measuredaccording to the BET volumetric method (British Standard BS4359/1-1984), of at least 100 m²/g, in particular of at least 180 m²/g,values of at least 220 m²/g being the most favourable. The SS isgenerally at most 800 m²/g, more specifically at most 700 m²/g, valuesof at most 650 m²/g being the most common.

The support generally exhibits a crystallization temperature of at least700° C., such as, for example, of at least 1000° C. The crystallizationtemperature of the support is determined by subjecting a sample of thesupport to a heat treatment at different temperatures and by examiningthe sample by X-ray diffraction after each heat treatment.

In addition, the pore volume of the support (PV) is usually at least 1.5cm³/g, values of at least 1.7 m³/g being recommended. The PV isgenerally at most 5 cm³/g, in particular at most 4.5 cm³/g, values of atmost 4 cm³/g being common. The pore volume (Pv) is the sum of the porevolume composed of the pores with a radius of less than or equal to 75A, measured by the nitrogen penetration method (BET method) (BritishStandard BS 4359/1-1984), and of the pore volume measured by the mercurypenetration method by means of a porosimeter of Poro 2000 type, sold byCarlo Erba Co. (Belgium Standard NBN B 05-202-1976).

The supports used generally exhibit an SS and a PV, expressedrespectively in m²/g [lacuna] in cm³/g, such that SS<(PV×564−358).Preferred supports exhibit an SS and a PV such that SS <(PV×682−542).Supports which exhibit an SS and a PV such that SS≦(PV×682−573) areparticularly preferred.

The support as described above can advantageously be obtained accordingto the process disclosed in Patent Applications WO 94/26790 andEP-A-0,712,868, which consists in mixing, in a first stage, an alcohol,water, a silicon alkoxide and an acid in amounts such that thewater/silicon molar ratio is from 2 to 50, in adding, to the hydrolysismedium thus obtained, in a second stage, an acidic solution of analuminium compound and/or a solution of a source of phosphate ions and,in a third stage, a precipitating agent, in order to obtain aprecipitate, which is washed, in a fourth stage, with water and thenwith an organic liquid, and dried, in a fifth stage, by distillationuntil a powder is obtained, which powder is calcined. Another processfor the preparation of the homogeneous and amorphous support consists inmixing, in a first stage, a source of silica, chosen from aqueousalkaline silica sols and aqueous alkaline inorganic silicate solutions,with a source of phosphate ions with a pH of less than 5, the pH of themedium being maintained at less than 5 throughout the duration of thefirst stage, in adding, in a second stage, an aluminium compound to themedium resulting from the first stage, and in forming, in a third stage,a precipitate by adding a precipitating agent to the medium of thesecond stage, the pH of the precipitation medium being maintained atgreater than or equal to 5 throughout the duration of the third stage,and in drying the precipitate until a powder is obtained, and in thencalcining the powder.

The catalytic solid which has given good results in the processaccording to the invention comprises from 0.05 to 10% by weight ofchromium and the support comprises silica (X), alumina (Y) and aluminiumphosphate (Z) in an (X):(Y):(Z) molar percentage of (10 to 95):(1 to80):(1 to 85) and more particularly in an (X):(Y):(Z) molar percentageof (20 to 80):(0.5 to 60):(5 to 60).

The polymerization process according to the invention is carried out inthe presence of a cocatalyst chosen from organoboron compounds.Organoboron compounds which are particularly well suited aretrialkylboranes and more particularly those in which the alkyl chainscomprise from 1 to 12 carbon atoms, preferably from 2 to 6 carbon atoms.Triethylborane gives good results.

The amount of cocatalyst employed is generally from 0.02 to 50 mmol perliter of solvent, of diluent or of reactor volume.

The amounts of solid catalyst and of cocatalyst employed in the processaccording to the invention are generally such that the molar ratio ofthe organoboron compound to the chromium present in the catalytic solidis at least 0.1 and more particularly at least 0.8. In addition, thisratio is favourably at most 20 and more particularly at most 10.

The polymerization process according to the invention can be carried outaccording to any known single-stage process. It is preferably carriedout in suspension in an aromatic, cycloaliphatic or aliphatichydrocarbon-comprising diluent at a temperature such that at least 80%by weight of the copolymer formed is insoluble in this diluent. Thediluent used is preferably a linear alkane, such as n-butane, n-hexaneor n-heptane, or a branched alkane, such as isobutane, isopentane or2,2-dimethylpropane. The diluent is preferably isobutane.

The polymerization temperature is generally at least 55° C., preferablyat least 65° C. The polymerization temperature generally does not exceed150° C. and preferably does not exceed 110° C.

The ethylene pressure in the polymerization reactor is generally chosenbetween atmospheric pressure and 5 MPa. The ethylene pressure ispreferably at least equal to 0.3 MPa and more particularly at leastequal to 0.4 MPa. The ethylene pressure generally does not exceed 2 MPaand more particularly does not exceed 1.5 MPa. In the specific casewhere the polymerization is carried out in suspension in a diluent, thepressure of the ethylene is generally adjusted so that the concentrationof ethylene in this diluent is at least 1 molar %, preferably at least 3molar %. The pressure of the ethylene is generally adjusted so that theconcentration of ethylene in the diluent does not exceed 20 molar % andpreferably does not exceed 10 molar %.

The amount of alpha-olefin employed in the polymerization processaccording to the invention depends on the amount of alpha-olefin desiredin the copolymer. It has been observed that, in the polymerizationprocess according to the invention, even without addition ofalpha-olefins to the polymerization medium, ethylene copolymerscomprising a small amount of units derived from alpha-olefins have beenencountered. This effect might be explained by the in situ production ofsmall amounts of alpha-olefins comprising at least 4 carbon atoms, whichwould be incorporated in the growing polymerization chain. The processaccording to the invention can consequently be carried out withoutaddition of alpha-olefins to the polymerization reactor. However, andmore particularly in the case where copolymers which are richer inalpha-olefins are desired, alpha-olefins as defined above are added tothe polymerization reactor. In this case, the amount of alpha-olefinsemployed is generally adjusted so that the alpha-olefin/ethylene molarratio is at least 0.005. The amount of alpha-olefin is preferablyadjusted so that this ratio is at least 0.01 and more particularly atleast 0.03. The amount of alpha-olefin is usually adjusted so that thealpha-olefin/ethylene molar ratio does not exceed 0.8. This ratiopreferably does not exceed 0.5 and more particularly does not exceed0.4.

The polymerization process according to the invention can be carried outin the presence of a transfer agent, such as, for example, hydrogen.

The process according to the invention makes it possible to obtain ahigh catalytic activity and, in addition, it makes it possible to obtaincopolymers having a low content of oligomers.

The process according to the invention has the advantage of exhibiting avery low, indeed even zero, induction period.

The copolymers according to the invention are well suited to the meltshaping of highly varied articles. To this end, the copolymers aregenerally mixed with the usual additives for processing polyolefins,such as stabilizers (antioxidizing agents and/or UV stabilizers),antistatic agents and processing aids, as well as pigments.

The invention consequently also relates to a composition comprising acopolymer according to the invention and at least one of the additivesdescribed above. The content of additives in the composition is ingeneral less than 10 and generally less than 5 parts per 100 parts byweight of copolymer.

Compositions comprising at least 95%, preferably at least 99%, by weightof a copolymer are particularly preferred.

The examples which follow are intended to illustrate the invention.

The meaning of the symbols used in these examples and the unitsexpressing the quantities mentioned and the methods for measuring thesequantities are explained belo

α=catalytic activity, expressed as g of copolymer obtained per hour andper g of catalyst employed, divided by the concentration of the ethylenein the isobutane.

FO=content of oligomers in the polymer, expressed as g of oligomers perkg of copolymer and measured by extraction with hexane at its boilingtemperature.

Q=content of hexene in the copolymer, measured as described above, thecontent of units derived from 1-hexene being calculated from themeasurements of the integrals of the lines characteristic of the unitsderived from 1-hexene (23.4, 34.9 and 38.1 ppm) with respect to theintegral of the line characteristic of the units derived from ethylene(30 ppm).

ESCR-A=the resistance to slow cracking is measured according to ASTMStandard D 1693 and is expressed in hours.

The other properties, HLMI, SD, μ₀/μ₂ and DS, were explained in thedescription.

EXAMPLE 1 In Accordance With the Invention

In this example, a catalytic solid comprising 0.7% by weight of chromiumon a support comprising 50.4 molar % of SiO₂, 1.8 molar % of Al₂O₃ and47.8 molar % of AlPO₄ and exhibiting a specific surface of 293 m²/g anda pore volume of 3.17 cm³/g was employed. The preparation of such asupport has been disclosed in Examples 1, 4 and 5 of Patent ApplicationEP-A-0,712,868. The catalytic solid was prepared by mixing the supportwith chromium acetylacetonate, by then treating the mixture in afluidized bed at 150° C. for 2 hours while flushing with air, and bysubsequently calcining it in the fluidized bed at 595° C. for 10 h underdry air.

The polymerization was carried out in isobutane in the presence of thiscatalytic solid and an amount of triethylborane (TEB), so as to obtainthe TEB/Cr molar ratio given in Table I. The temperature was raised andethylene was introduced into the reactor, so as to obtain aconcentration in the isobutane given in Table I. An amount of 1-hexenewas introduced, so as to obtain a 1-hexene/ethylene molar ratio given inTable I. This ethylene/hexene molar ratio was kept constant throughoutthe duration of polymerization. After the polymerization, the copolymerwas recovered.

The polymerization conditions, the results obtained and the propertiesof the copolymer are collated in Table I.

The copolymer obtained in Example 1 was fractionated and the hexenecontents in different fractions having different molecular masses weremeasured. The graph in FIG. 1 gives the hexene contents (on theordinate), expressed in g/kg, of the different fractions (on theabscissa). It is seen from this that the fractions with molecular massesvarying from 5000 to 500,000 each comprise a hexene content of 16 to 20g/kg and that the fractions with molecular masses varying from 100,000to 500,000 comprise a hexene content of approximately 16 g/kg.

EXAMPLE 2 In Accordance With the Invention

In this example, the polymerization was carried out in isobutane with acatalytic solid comprising 0.7% by weight of chromium on a supportcomprising 52.3 molar % of SiO₂, 2.2 molar % of Al₂O₃ and 45.5 molar %of AlPO₄ and exhibiting a specific surface of 297 m²/g and a pore volumeof 2.2 cm³/g. The catalytic solid was prepared by mixing the supportwith chromium acetylacetonate, by then treating the mixture in afluidized bed at 150° C. for 2 hours while flushing with air and bysubsequently calcining it in the fluidized bed at 595° C. for 10 h underdry air.

The polymerization conditions, the results obtained and the propertiesof the copolymer are collated in Table I.

EXAMPLES 3 and 4 In Accordance With the Invention

n these examples, the polymerization was carried out in isobutane withthe catalytic solid described in Example 1, except that it was calcinedat 700° C. for 16 h under dry air.

The polymerization conditions, the results obtained and the propertiesof the copolymer are collated in Table I.

EXAMPLE 5R Not in Accordance With the Invention

This example was carried out by means of the catalytic solid describedin Example 1 but in the absence of triethylborane. The properties of thecopolymer obtained are collated in Table I. It is seen from this thatthe copolymer obtained exhibits, for an alpha-olefin content of 11 g/kg,a lower SD than the copolymer of Example 1, which comprises morecopolymer.

EXAMPLE 6R and 7R Not in Accordance With the Invention

These examples were carried out by means of a conventional Cr catalyst,comprising 1% of chromium on a silica support (Cogel 963, sold by thecompany Grace), instead of the catalytic solid of Example 1. Theproperties of the copolymer obtained are collated in Table I. It is seenfrom this that the copolymer obtained in Example 6R exhibits, for acomparable SD, an alpha-olefin content which is markedly lower than thatof the copolymers of Examples 1 and 4, as well as a narrowerdistribution of the molecular masses (μ₀/μ₂ ratio).

Table I also shows that the copolymer obtained in Example 7R exhibits,for a comparable SD, an alpha-olefin content which is markedly lowerthan that of the copolymer of Example 2 and consequently exhibits apoorer resistance to slow cracking.

TABLE 1 Example 1 2 3 4 5R 6R 7R TEB/Cr (mol/mol) 1 0.89 2 2 0 0 0Ethylene/isobutane 7 8 9 9 7 7 10 (molar %) Polymerization T 82 94 80 7097 94 92 (° C.) Hexene/ethylene 0.263 0.041 0.10 0.30 0.051 0.04 0.011(mol/mol) α (g/g cata/h) 6660 8000 2840 2560 6500 3000 3800 FO (g/kg) 3420 28 30 15 11 — Q (g/kg) 16 8 7 14 11 5 1 HLMI (g/10 min) 26.3 5.7 2118.1 11.2 10.8 3.6 SD (kg/m³) 949.7 954.4 956.6 950.7 948.5 951.2 954μ₀/μ₂ — — 16 17.4 — 10.04 — DS — — 2.2 2.3 — 1.65 — ESCR-A (h) — >500 —— — — 450

What is claimed is:
 1. A copolymer of ethylene and of at least onealpha-olefin obtained by means of a catalytic solid based on chromiumoxide, the said copolymer having a melt flow index HLMI, expressed ing/10 min, an amount of alpha-olefin Q, expressed in g of alpha-olefinper kg of copolymer, a standard density SD, expressed in kg/m³, anddynamic viscosities μ₀ and μ₂ measured at 190° C., respectively at rategradients of 1 and 100 s⁻¹, corresponding to the relationshipsSD≧(952.75+5.40×log HLMI−0.79×Q), and μ₀/μ₂≧(23.67−6.67×log HLMI). 2.The copolymer according to claim 1, wherein the SD is at least equal to(952.75+5.40×log HLMI−0.79×Q).
 3. The copolymer according to claim 1,wherein the alpha-olefin comprises from 3 to 12 carbon atoms.
 4. Thecopolymer according to claim 3, wherein the alpha-olefin is 1-buteneand/or 1-hexene.
 5. The copolymer according to claim 1, wherein theamount of alpha-olefin Q is at least equal to 1 g/kg and does not exceed100 g/kg of copolymer.
 6. The copolymer according to claim 1, whereinthe SD is greater than 935 kg/m³ and does not exceed 965 kg/m³.
 7. Thecopolymer according to claim 1, wherein the HLMI is at least equal to0.1 g/10 min and does not exceed 100 g/10 min.
 8. The copolymeraccording to claim 1, wherein the ratio μ₀/μ₂ is at least equal to 10and does not exceed
 50. 9. The copolymer according to claim 1, saidcopolymer having a die swell (DS) of at least 1.3.
 10. A process for thepreparation of ethylene copolymer in accordance with claim 1, whereinethylene and at least one alpha-olefin are polymerized in a singlestage, optionally in the presence of hydrogen, in the presence of acatalytic solid based on chromium oxide deposited on a homogenous andamorphous support, the said support comprising silica, alumina andaluminum phosphate, said support being obtained by co-precipitation ofprecursor components of said support and a co-catalyst chosen fromorganoboron compounds, wherein the catalytic solid comprises from 0.05to 10% by weight of chromium and the support comprises silica (X),alumina (Y) and aluminum phosphate (Z) in an (X):(Y):(Z) molarpercentage of (10 to 95):(1 to 80):(1 to 85).
 11. The process accordingto claim, 10 wherein the catalytic solid comprises from 0.05 to 10% byweight of chromium and the support comprises silica (X), alumina (Y) andaluminium phosphate (Z) in an (X):(Y):(Z) molar percentage of (20 to80):(0.5 to 60):(5 to 60).
 12. The process according to claim 10,wherein the organoboron compound is a trialkylborane in which the alkylchains comprise from 1 to 12 carbon atoms.
 13. The process according toclaim, 12 wherein the trialklborane is triethylborane.
 14. A compositioncomprising at least one additive selected from the group consisting ofstabilizers, antioxidizing agents, processing aids and pigments and atleast 95% by weight of a copolymer in accordance with claim
 1. 15. Anmelt-shaped article comprising a composition according to claim
 14. 16.The process of claim 10 wherein the support is a coprecipiate formed byprecipitating an admixture comprising a support source selected from thegroup consisting of silicon alkoxide, a source of aluminum and a sourceof phosphate.
 17. The process of claim 16, wherein the precipitate isproduced by mixing a silicon alkoxide and an acid in a hydrolysismedium; adding to the hydrolysis medium a source of phosphate ions; andadding a precipitating reagent to the phosphate containing hydrolysismedium.
 18. The process of claim 16, wherein the precipitate is formedby mixing an inorganic silicate source with a source of phosphate at apH of less than 5 and adding a precipitating agent at a pH of greaterthan 5.